1. Field of the Invention
The present invention relates to a lens, and more particularly to a zoom lens.
2. Description of Related Art
Referring to FIG. 1, a zoom lens 100 disclosed in U.S. Pat. No. 5,329,402 includes a first lens group 110 and a second lens group 120. The first lens group 110 is composed of three lenses 112, 114, and 116, and the second lens group 120 is composed of five lenses 122, 124, 126, 128, and 129. Since an f-number of the zoom lens 100 is between 3.6 and 4.6, an aperture of the zoom lens 100 is small, and it is difficult to achieve a larger relative aperture.
Based on the above, in the conventional art, in order to achieve a small f-number of the zoom lens and to keep a good imaging quality, an aspheric lens is usually used for eliminating image aberration. The zoom lens 200 in FIG. 2 disclosed in the U.S. Pat. No. 5,541,772 includes a first lens group 210 and a second lens group 220. The first lens group 210 is composed of three lenses 212, 214, and 216, and the second lens group 120 is composed of two lenses 222 and 224. In order to achieve the smaller f-number of the zoom lens 200 while keeping a good imaging quality, the lenses 214 and 216 in the first lens group 210 and the lens 222 in the second lens group 220 are all aspheric lenses. However, the aspheric lenses are expensive, so the manufacturing cost of the zoom lens 200 is very high.